I asked PS again about multi account play this time regarding private MTT's. Now it's clear:
// quote
Hello Jörg,
Thank you for your email.
Please note that you can both play in MTTs together, and if you happen to
be at the same table, then you are allowed to play lightly against each
other.
You must play against the people you know in the same way you would play
against rest of players at the table.
Please do not hesitate to contact us should you require any further
assistance.
Kind Regards,
Elisa
PokerStars Support Team
// end quote
So I won't hesitate to play THF next time
